1. Field of the Invention
The present invention relates to a method for evaluating a faulty point occurring in the middle of a multi-stage optical amplifying and repeating transmission line, particularly a transmission line of an optical submarine cable engaged in multi-stage optical amplifying and repeating operations, an apparatus for evaluating a faulty point, and a system for evaluating a faulty point. Note that below, an optical submarine cable using an optical fiber as a signal transmission line will be explained as a preferable example.
2. Description of the Related Art
As is well known, optical submarine cables are used for extremely long distance optical transmission of as much as several 1000 km for example for crossing the oceans. Such extremely long distance optical submarine cables have a high probability of breakage and other faults due to various factors. In addition, searching for a faulty point in that very long distance, in particular searching for it quickly, is becoming increasingly difficult. However, to provide a high level of quality of service, taking several hours or another long time for searching for a fault is unacceptable.
Therefore, in the past, various techniques have been proposed for searching for faults. For example, there are the art described in Japanese Unexamined Patent Publication (Kokai) No. 5-327621 and Japanese Unexamined Patent Publication (Kokai) No. 7-240718. In general, a multi-stage optical amplifying and repeating submarine cable is comprised of an optical signal system for multi-stage optical amplifying and repeating an optical signal as it is and a power supply system for giving optical amplifying energy to the optical signal system. Japanese Unexamined Patent Publication (Kokai) No. 5-327621 proposes a new way to search for faults through the power supply system, while Japanese Unexamined Patent Publication (Kokai) No. 7-240718 proposes a new way to search for faults through the optical signal system.
A typical related art searching for a fault through the optical signal system transmits an optical command from at least one of the units of terminal equipment provided at the two ends of the submarine cable to each of the large number of optical amplifying repeaters inserted in the optical submarine cable one by one in order from the nearest one on out, confirms an optical response to this one at a time, and, if no optical response is received, estimates that the fault has occurred from there.
Such a command response system for searching for a fault has a high precision since it is a so-called “pinpoint” type of search. However, even if performing this search automatically using software, several seconds to several tens of seconds are required for the command response operation for each optical amplifying repeater, so sometimes a tremendous amount of time of several hours ends up being taken. This makes this method unsuitable for the “high speed fault search” aimed at by the present invention.
On the other hand, a typical related art searching for a fault through the power supply system supplies power to the two ends or one end of the optical submarine cable from power supply apparatuses provided together with the units of terminal equipment at the two ends of the cable and judges a fault location from the change in the power supply voltage occurring along with a fault in the middle of the optical submarine cable by the direct current resistance (DCR) method. (See the later explained FIG. 10.)
The DCR method does not require any software as explained above, has an extremely short fault search time, and thereby matches with the “high speed fault search” aimed at by the present invention.
However, with the DCR method utilizing the power supply system, there are the problems that (i) from the nature of the DCR method, detection of faults occurring near the midpoint of an optical submarine cable is difficult and (ii) with a fault occurring at an optical submarine cable, the DC resistance will fluctuate depending on the state of the fault, for example, the location of breakage of the optical submarine cable, and therefore stable identification of the faulty point will not be possible. That is, the DCR method enables high speed fault searches, but it is difficult to get high speed fault searches which always give reliable and stable results for faults occurring at an optical submarine cable.